Concurrent prediction of branch addresses and update of register contents

ABSTRACT

A value to be used in register-indirect branching is predicted and concurrently stored in a selected location accessible to one or more instructions. The value may be a target address used by an indirect branch and the selected location may be a hardware register, providing concurrent prediction of branch addresses and the update of register contents.

BACKGROUND

One or more aspects relate, in general, to processing within a computingenvironment, and in particular, to facilitating such processing.

Many computing systems use register-indirect branching, in which alocation of the address of the next instruction to execute is specifiedin a branch instruction, instead of the address itself. For instance, alocation of a register that includes the address is specified.

Further, in accordance with commonly used application binary interfaces(ABIs), a branch address is first loaded into a general purpose register(GPR), and then, transferred to a special purpose control register (SPR)before effecting a register-indirect branch. For instance, in the PowerInstruction Set Architecture (ISA), offered by International BusinessMachines Corporation, Armonk, N.Y., a branch instruction branches to acounter (CTR) special purpose register. However, the special purposeregister is not loaded directly, but via a general purpose register.

The counter register tends to be expensive to read-out. Therefore, atleast one ABI indicates that the value of CTR is to be stored in anotherregister, such as R12, when a branch to a subroutine (BCTR) isperformed, enabling the other register to be used as a base register bythe called function. However, when a branch prediction is made, thebranch address may be predicted before the R12 value has been loaded,making the called subroutine stall responsive to a data access, andlimiting performance.

SUMMARY

Shortcomings of the prior art are overcome and additional advantages areprovided through the provision of a computer program product forfacilitating processing within a computing environment. The computerprogram product comprises a storage medium readable by a processingcircuit and storing instructions for performing a method. The methodincludes, for instance, predicting, using a processor of the computingenvironment, a predicted value to be used in register-indirectbranching. The predicted value is stored in a selected locationaccessible to one or more instructions of the computing environment, andthe storing is performed concurrently to processing a register-indirectbranch. The selected location is in addition to another location used tostore an instruction address. The predicted value is used in speculativeprocessing that includes the register-indirect branch.

By concurrently processing a register-indirect branch and storing apredicted value resulting from that processing in a location (inaddition to a program counter) accessible to other instructions,processing is facilitated and performance may be enhanced, particularlyif the predicted value is correct.

As one or more examples, the predicted value includes a target addressto be used by an instruction fetch, and the selected location includes ahardware register. Further, in one example, the using includesredirecting the instruction fetch to the target address.

In a further aspect, a determination is made as to whether the predictedvalue is accurate. Based on determining the predicted value isinaccurate, recovery is performed.

The recovery includes, for instance, flushing one or more instructionsexecuted subsequent to the using, and replacing the predicted value witha non-predicted value. In one example, the replacing includes using oneor more rename registers to provide the non-predicted value.

In one embodiment, the storing includes allocating a register to holdthe predicted value, and copying the predicted value to the registerallocated to hold the predicted value.

As one example, the register is a rename register allocated for alogical register assigned to hold the predicted value. Further, in oneembodiment, the rename register is marked as available.

Computer-implemented methods and systems relating to one or more aspectsare also described and claimed herein. Further, services relating to oneor more aspects are also described and may be claimed herein.

Additional features and advantages are realized through the techniquesdescribed herein. Other embodiments and aspects are described in detailherein and are considered a part of the claimed aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more aspects are particularly pointed out and distinctly claimedas examples in the claims at the conclusion of the specification. Theforegoing and objects, features, and advantages of one or more aspectsare apparent from the following detailed description taken inconjunction with the accompanying drawings in which:

FIG. 1A depicts one example of a computing environment to incorporateand use one or more aspects of the present invention;

FIG. 1B depicts further details of a processor of FIG. 1A, in accordancewith one or more aspects of the present invention;

FIG. 1C depicts further details of one example of an instructionexecution pipeline used in accordance with one or more aspects of thepresent invention;

FIG. 1D depicts further details of one example of a processor of FIG.1A, in accordance with an aspect of the present invention;

FIG. 2 depicts one example of processing associated with prediction fora register-indirect branch, in accordance with an aspect of the presentinvention;

FIG. 3 depicts one example of checking prediction correctness, inaccordance with an aspect of the present invention;

FIG. 4 depicts another example of checking prediction correctness, inaccordance with an aspect of the present invention;

FIG. 5 depicts one example of processing associated with prediction fora register-indirect branch and an affiliated register, in accordancewith an aspect of the present invention;

FIG. 6 depicts another example of checking prediction correctness, inaccordance with an aspect of the present invention;

FIG. 7 depicts an example of checking prediction correctness foraffiliated registers, in accordance with an aspect of the presentinvention;

FIG. 8 depicts one example of processing associated with externallyindicated affiliated register prediction (EIARP), in accordance with anaspect of the present invention;

FIG. 9 depicts one example of processing associated with a contextswitch and externally indicated affiliated register prediction, inaccordance with an aspect of the present invention;

FIG. 10 depicts one example of processing associated with enteringhardware exception/interrupt processing, in accordance with an aspect ofthe present invention;

FIG. 11 depicts one example of processing associated with exitinghardware exception/interrupt processing, in accordance with an aspect ofthe present invention;

FIG. 12 depicts one example of processing associated with prediction fora register-indirect branch and the use of externally indicatedaffiliated register prediction, in accordance with an aspect of thepresent invention;

FIG. 13 depicts one example of processing associated with prediction fora register-indirect branch and for predicting whether an affiliatedregister may be predicted, in accordance with an aspect of the presentinvention;

FIG. 14 depicts another example of checking prediction correctness, inaccordance with an aspect of the present invention;

FIG. 15 depicts another example of checking prediction correctness foraffiliated registers, in accordance with an aspect of the presentinvention;

FIG. 16 depicts yet another example of checking prediction correctnessfor affiliated registers, in accordance with an aspect of the presentinvention;

FIG. 17 depicts one example of processing associated withregister-indirect branch prediction and for dynamically selecting anaffiliated register that may be predicted, in accordance with an aspectof the present invention;

FIG. 18 depicts an example of checking prediction correctness foraffiliated registers and updating a predictor, in accordance with anaspect of the present invention;

FIG. 19 depicts one example of processing associated with recognizing afusion sequence, in accordance with an aspect of the present invention;

FIG. 20 depicts one example of processing associated with externallyindicated affiliated register prediction and a fusion-based affiliationsequence, in accordance with an aspect of the present invention;

FIG. 21 depicts one example of processing associated with performingbranch prediction with a fusion-based affiliated sequence, in accordancewith an aspect of the present invention;

FIG. 22 depicts one example of processing associated with determiningwhether an instruction is affiliation creating or destroying, inaccordance with an aspect of the present invention;

FIG. 23 depicts one example of processing associated with predictingaffiliated derived registers, in accordance with an aspect of thepresent invention;

FIG. 24 depicts one example of checking prediction correctness foraffiliated derived registers, in accordance with an aspect of thepresent invention;

FIG. 25 depicts one example of processing associated with branchprediction and fusion-based affiliated derived sequences, in accordancewith an aspect of the present invention;

FIGS. 26A-26B depict one embodiment of facilitating processing within acomputing environment, in accordance with an aspect of the presentinvention;

FIG. 27A depicts another example of a computing environment toincorporate and use one or more aspects of the present invention;

FIG. 27B depicts further details of the memory of FIG. 27A;

FIG. 28 depicts one embodiment of a cloud computing environment; and

FIG. 29 depicts one example of abstraction model layers.

DETAILED DESCRIPTION

In accordance with one or more aspects, based on predicting a value ofcontents of a register (e.g., a target address of a target addressregister) to be used for register-indirect branches, the predicted valueis stored in the register (e.g., the target address register, alsoreferred to herein as a predicted register). The register in thenavailable for future use. For instance, the contents of a target addressregister are updated and made available in conjunction with a predictiveinstruction fetch. When a target address is predicted, it isconcurrently made available for both instruction fetch and for storagein a register or other selected location accessible to others (e.g.,other instructions or operations). This allows others to use thepredicted value.

As an example, the predicted register or location is a register orlocation other than a program counter that is automatically updatedbased on a prediction. It is, for instance, a register or location inaddition to the program counter, such as a counter (CTR) register orother selected register/location. The target address, when predicted, isconcurrently made available to the predicted register, in that onefacility processes the register-indirect branch and stores the predictedtarget address. For instance, it is within the confines of onearchitected instruction (e.g., at the hardware/software interface).Another architected instruction is not needed to copy or move the valueto the predicted register/location.

In a further aspect, in addition to predicting the value of contents ofa register and storing the predicted value in the predicted register,the predicted value is also stored in another location, such as anotherregister used to facilitate processing. This other location or registeris referred to herein as an affiliated location or an affiliatedregister that stores the predicted value, which is used by otherinstructions. The affiliated register is one that is associated with thepredicted register. For instance, an affiliated register has a knownrelationship with another register (e.g., the predicted register), suchas being known as a copy of the other register. In one particularinstance, both registers may be referenced by an instruction. As anexample, the MTCTR R12 instruction moves the contents of R12 to the CTRregister. Thus, R12 is affiliated with CTR. Other such examples exist.

In yet a further aspect, a control is provided to specify whether anaffiliated register is to be used in prediction. If an affiliatedregister is to be predicted, the control also provides an indication ofthe affiliated register (e.g., a register number). This control is, forinstance, code specific, and may be enabled/disabled for each unit ofcode, such as an application, process, function, module, or dynamicallyshared object (e.g., library), as examples.

In still a further aspect, the predicted value may be used to predict avalue to be stored in an affiliated derived register (or other location)to further facilitate processing. An affiliated derived register is onein which its contents are derived from the contents of an affiliatedregister. For instance, the contents of R12 may be used, along with anoffset, to obtain a value to be stored in a selected register, such asR2. Thus, R2, in this example, is an affiliated derived register. Otherexamples also exist.

Additionally, in one or more aspects, a particular sequence ofinstructions used in register-indirect branching and specifying one ormore registers that may be predicted may be recognized, and basedthereon, a sequence of operations performing operations of theparticular sequence of instructions are fused to facilitate processing.That is, a fusion process is performed to combine one or moreinstructions into a single composite process of operations. That is, thesequence of instructions is to be treated as a whole.

Various aspects are described herein. Further, many variations arepossible without departing from a spirit of aspects of the presentinvention. It should be noted that various aspects and features aredescribed herein, and unless otherwise inconsistent, each aspect orfeature may be combinable with any other aspect or feature.

One embodiment of a computing environment to incorporate and use one ormore aspects of the present invention is described with reference toFIG. 1A. In one example, the computing environment is based on thez/Architecture, offered by International Business Machines Corporation,Armonk, N.Y. One embodiment of the z/Architecture is described in“z/Architecture Principles of Operation,” IBM Publication No.SA22-7832-10, March 2015, which is hereby incorporated herein byreference in its entirety. Z/ARCHITECTURE is a registered trademark ofInternational Business Machines Corporation, Armonk, N.Y., USA.

In another example, the computing environment is based on the PowerArchitecture, offered by International Business Machines Corporation,Armonk, N.Y. One embodiment of the Power Architecture is described in“Power ISA™ Version 2.07B,” International Business Machines Corporation,Apr. 9, 2015, which is hereby incorporated herein by reference in itsentirety. POWER ARCHITECTURE is a registered trademark of InternationalBusiness Machines Corporation, Armonk, N.Y., USA.

The computing environment may also be based on other architectures,including, but not limited to, the Intel x86 architectures. Otherexamples also exist.

As shown in FIG. 1A, a computing environment 100 includes, for instance,a computer system 102 shown, e.g., in the form of a general-purposecomputing device. Computer system 102 may include, but is not limitedto, one or more processors or processing units 104 (e.g., centralprocessing units (CPUs)), a memory 106 (referred to as main memory orstorage, as examples), and one or more input/output (I/O) interfaces108, coupled to one another via one or more buses and/or otherconnections 110.

Bus 110 represents one or more of any of several types of busstructures, including a memory bus or memory controller, a peripheralbus, an accelerated graphics port, and a processor or local bus usingany of a variety of bus architectures. By way of example, and notlimitation, such architectures include the Industry StandardArchitecture (ISA), the Micro Channel Architecture (MCA), the EnhancedISA (EISA), the Video Electronics Standards Association (VESA) localbus, and the Peripheral Component Interconnect (PCI).

Memory 106 may include, for instance, a cache 120, such as a sharedcache, which may be coupled to local caches 122 of processors 104.Further, memory 106 may include one or more programs or applications130, an operating system 132, and one or more computer readable programinstructions 134. Computer readable program instructions 134 may beconfigured to carry out functions of embodiments of aspects of theinvention.

Computer system 102 may also communicate via, e.g., I/O interfaces 108with one or more external devices 140, one or more network interfaces142, and/or one or more data storage devices 144. Example externaldevices include a user terminal, a tape drive, a pointing device, adisplay, etc. Network interface 142 enables computer system 102 tocommunicate with one or more networks, such as a local area network(LAN), a general wide area network (WAN), and/or a public network (e.g.,the Internet), providing communication with other computing devices orsystems.

Data storage device 144 may store one or more programs 146, one or morecomputer readable program instructions 148, and/or data, etc. Thecomputer readable program instructions may be configured to carry outfunctions of embodiments of aspects of the invention.

Computer system 102 may include and/or be coupled toremovable/non-removable, volatile/non-volatile computer system storagemedia. For example, it may include and/or be coupled to a non-removable,non-volatile magnetic media (typically called a “hard drive”), amagnetic disk drive for reading from and writing to a removable,non-volatile magnetic disk (e.g., a “floppy disk”), and/or an opticaldisk drive for reading from or writing to a removable, non-volatileoptical disk, such as a CD-ROM, DVD-ROM or other optical media. Itshould be understood that other hardware and/or software componentscould be used in conjunction with computer system 102. Examples,include, but are not limited to: microcode, device drivers, redundantprocessing units, external disk drive arrays, RAID systems, tape drives,and data archival storage systems, etc.

Computer system 102 may be operational with numerous other generalpurpose or special purpose computing system environments orconfigurations. Examples of well-known computing systems, environments,and/or configurations that may be suitable for use with computer system102 include, but are not limited to, personal computer (PC) systems,server computer systems, thin clients, thick clients, handheld or laptopdevices, multiprocessor systems, microprocessor-based systems, set topboxes, programmable consumer electronics, network PCs, minicomputersystems, mainframe computer systems, and distributed cloud computingenvironments that include any of the above systems or devices, and thelike.

Further details regarding one example of processor 104 are describedwith reference to FIG. 1B. Processor 104 includes a plurality offunctional components used to execute instructions. These functionalcomponents include, for instance, an instruction fetch component 150 tofetch instructions to be executed; an instruction decode unit 152 todecode the fetched instructions and to obtain operands of the decodedinstructions; instruction execution components 154 to execute thedecoded instructions; a memory access component 156 to access memory forinstruction execution, if necessary; and a write back component 160 toprovide the results of the executed instructions. One or more of thesecomponents may, in accordance with one or more aspects of the presentinvention, be used to execute one or more instructions and/or operationsassociated with predictive instruction fetch and/or processing 166.

Processor 104 also includes, in one embodiment, one or more registers168 to be used by one or more of the functional components. Processor104 may include additional, fewer and/or other components than theexamples provided herein.

Further details regarding an execution pipeline of processor 104 aredescribed with reference to FIG. 1C. Although various processing stagesof the pipeline are depicted and described herein, it will be understoodthat additional, fewer and/or other stages may be used without departingfrom the spirit of aspects of the invention.

Referring to FIG. 1C, in one embodiment, an instruction is fetched 170from an instruction queue, and branch prediction 172 and/or decoding 174of the instruction may be performed. The decoded instruction may beadded to a group of instructions 176 to be processed together. Thegrouped instructions are provided to a mapper 178 that determines anydependencies, assigns resources and dispatches the group ofinstructions/operations to the appropriate issue queues. There are oneor more issue queues for the different types of execution units,including, as examples, branch, load/store, floating point, fixed point,vector, etc. During an issue stage 180, an instruction/operation isissued to the appropriate execution unit. Any registers are read 182 toretrieve its sources, and the instruction/operation executes during anexecute stage 184. As indicated, the execution may be for a branch, aload (LD) or a store (ST), a fixed point operation (FX), a floatingpoint operation (FP), or a vector operation (VX), as examples. Anyresults are written to the appropriate register(s) during a write backstage 186. Subsequently, the instruction completes 188. If there is aninterruption or flush 190, processing may return to instruction fetch170.

Further, in one example, coupled to the decode unit is a registerrenaming unit 192, which may be used in the saving/restoring ofregisters.

Additional details regarding a processor are described with reference toFIG. 1D. In one example, a processor, such as processor 104, is apipelined processor that may include prediction hardware, registers,caches, decoders, an instruction sequencing unit, and instructionexecution units, as examples. The prediction hardware includes, forinstance, a local branch history table (BHT) 105 a, a global branchhistory table (BHT) 105 b, and a global selector 105 c. The predictionhardware is accessed through an instruction fetch address register(IFAR) 107, which has the address for the next instruction fetch.

The same address is also provided to an instruction cache 109, which mayfetch a plurality of instructions referred to as a “fetch group”.Associated with instruction cache 109 is a directory 111.

The cache and prediction hardware are accessed at approximately the sametime with the same address. If the prediction hardware has predictioninformation available for an instruction in the fetch group, thatprediction is forwarded to an instruction sequencing unit (ISU) 113,which, in turn, issues instructions to execution units for execution.The prediction may be used to update IFAR 107 in conjunction with branchtarget calculation 115 and branch target prediction hardware (such as alink register prediction stack 117 a and a count register stack 117 b).If no prediction information is available, but one or more instructiondecoders 119 find a branch instruction in the fetch group, a predictionis created for that fetch group. Predicted branches are stored in theprediction hardware, such as in a branch information queue (BIQ) 125,and forwarded to ISU 113.

A branch execution unit (BRU) 121 operates in response to instructionsissued to it by ISU 113. BRU 121 has read access to a condition register(CR) file 123. Branch execution unit 121 further has access toinformation stored by the branch scan logic in branch information queue125 to determine the success of a branch prediction, and is operativelycoupled to instruction fetch address register(s) (IFAR) 107corresponding to the one or more threads supported by themicroprocessor. In accordance with at least one embodiment, BIQ entriesare associated with, and identified by an identifier, e.g., by a branchtag, BTAG. When a branch associated with a BIQ entry is completed, it isso marked. BIQ entries are maintained in a queue, and the oldest queueentries are deallocated sequentially when they are marked as containinginformation associated with a completed branch. BRU 121 is furtheroperatively coupled to cause a predictor update when BRU 121 discovers abranch misprediction.

When the instruction is executed, BRU 121 detects if the prediction iswrong. If so, the prediction is to be updated. For this purpose, theprocessor also includes predictor update logic 127. Predictor updatelogic 127 is responsive to an update indication from branch executionunit 121 and configured to update array entries in one or more of thelocal BHT 105 a, global BHT 105 b, and global selector 105 c. Thepredictor hardware 105 a, 105 b, and 105 c may have write ports distinctfrom the read ports used by the instruction fetch and predictionoperation, or a single read/write port may be shared. Predictor updatelogic 127 may further be operatively coupled to link stack 117 a andcount register stack 117 b.

Referring now to condition register file (CRF) 123, CRF 123 isread-accessible by BRU 121 and can be written to by the execution units,including but not limited to, a fixed point unit (FXU) 141, a floatingpoint unit (FPU) 143, and a vector multimedia extension unit (VMXU) 145.A condition register logic execution unit (CRL execution) 147 (alsoreferred to as the CRU), and special purpose register (SPR) handlinglogic 149 have read and write access to condition register file (CRF)123. CRU 147 performs logical operations on the condition registersstored in CRF file 123. FXU 141 is able to perform write updates to CRF123.

Processor 104 further includes, a load/store unit 151, and variousmultiplexors 153 and buffers 155, as well as address translation tables157, and other circuitry.

Processor 104 executes programs (also referred to as applications) thatuse hardware registers to store information. Programs that callroutines, such as functions, subroutines or other types of routines, areresponsible for saving registers used by the caller (i.e., the programscalling the routines) and for restoring those registers upon return fromthe callee (the called routine). Likewise, the callee is responsible forsaving/restoring registers that it uses, as shown in the code examplesprovided below.

Further, many computing systems use base registers for addressing dataand/or code. For example, System/360 (System z), offered byInternational Business Machines Corporation, uses a branch and linkregister (BALR) instruction and a USING directive to create an index foraddressing of data and code. As a further example, Power Systems use abranch and link (BL) instruction with an offset (e.g., BL. +4) to createa program counter (PC) address in a register to use for data addressingin position independent code (PIC). Other systems similarly use baseregisters for addressing data and code.

As indicated, different sequences have been used to establishaddressability. For instance, sequences have been used in which theprogram counter (PC) value is loaded in the callee function, or theprovided function call address is used. This offers the ability toprovide a known value represented by the current instruction address,but may use an expensive sequence. Conversely, using the specifiedfunction entry address results in a simpler sequence, but depends on apotentially long dependence chain to establish the entry point address.The following examples describe sequences previously used to establishaddressability.

One example of code for System/360 (System z) is depicted below: Thefollowing System/360 assembly code shows the structure of subroutinecalls commonly used to call a subroutine in mainframe systems based onthe System/360 architecture:

 ... 1 r15=A(subr) ! load address of subroutine subr in r15 balr r14,r15! save return address in r14 and set ... ! pc to contents of r15

The following System/360 (System z) assembly code shows the structure ofcalled subroutine calls in mainframe systems based on the System/360architecture:

subr: stm r14, r12, ! store multiple registers into caller's 12 (r13) !save area (r14, r15, r0,...,r12) balr r12, 0 ! set address in r12 foruse as base reg using *, r12 ! pseudo-op to tell assembler to use r12 lar11, savearea ! load address of my save area st r13, savearea+4 ! storeaddress of caller's save area st r11, 8 (r13) ! store address of my savearea in caller's save area ... body of subroutine... ... r0 is used forreturn code, if present ... 1 r13, savearea+4 ! load address of caller'ssave area lm r14, r12, ! load multiple registers from caller's 12 (r13)save area ! (r14, r15, r0, ..., r12) br r14 ! return to caller

Further, one example of code for a system based on the Powerarchitecture is depicted below. The following Power architectureassembly code shows the structure of subroutine calls commonly used tocall a subroutine in Power systems based on the Power architecture:

... lwz r15=A(subr) ! load address of subroutine subr in r15 mtctr r15! ctr = r15 bctrl ! save return address in lr and set ! pc to contentsof ctr ...

The following Power architecture assembly code shows the structure ofcalled subroutine calls in Power systems based on the Powerarchitecture:

subr: stmw r10, r4, ! store multiple registers into 12 (r13) caller'ssave area ! (r14, r15, r0, ..., r12) mflr r0 ! Save lr bl .+4 ! Get PCto LR base:mflr r12 ! Use address in r12 as base reg mtlr r0 ! RestoreLR ... body of subroutine ... lwz r11, (savearea-base) @1, ! load datafrom r12 Global ! Offset Table (GOT) ... r0 is used for return code, ifpresent ... blr ! return to caller

Loading the program counter as a base, as in the above examples, incursoverhead. For instance, for System/360, the code includes an extra balrr12, 0 instruction, and on Power Systems, there is an extra set ofinstructions, including mflr/b1.+4/mflr/mtlr. Thus, in one codetransformation, the function entry point is also used as a base, asshown below.

One example of System/360 code that includes the function entry point asa base is a follows:

... 1 r12=A(subr) ! load address of subroutine subr in r12 balr r14, r12! save return address in r14 and set ! pc to contents of r12 ... subr:using *, r12 ! pseudo-op to tell assembler to use r12 as base stm r14,r12, ! store multiple registers into caller's 12 (r13) save area ! (r14,r15, r0, ..., r12) la r11, savearea ! load address of my save area str13, savearea+4 ! store address of caller's save area st r11, 8 (r13) !store address of my save area in caller's save area ... body ofsubroutine ... ... r0 is used for return code, if present ... 1 r13,savearea+4 ! load address of caller's save area lm r14, r12, ! loadmultiple registers from caller's 12 (r13) save area !  (r14, r15, r0,..., r12) br r14 ! return to caller

One example of code based on the Power architecture is as follows:

... lwz r15=A(subr) ! load address of subroutine in r15 mtctr  r15 ! ctr= r15 bctrl ! save return address in lr and set ! pc to contents of ctr...  subr: base: stmw  r10, r4, ! store multiple registers into caller's 12 (r13) ! save area (r14, r15, r0, ..., r12) mfctr r12 ! move addressin ctr to r12 as base reg ... body of subroutine ... lwz r11,(savearea-base)@1, ! load data from r12 global ! offset table (GOT) ...r0 is used for return code, if present ... blr ! return to caller

While the above transformation allows the target address to be availablefor executing the branch to enter the subroutine, the address may not beavailable during speculative execution after a branch prediction untilthe time when the target address becomes available to validate theprediction. For instance, the address is not available, at the time ofprediction, in a register or other selected location (different from theprogram counter) accessible by other instructions or operations. Thus,in accordance with an aspect of the present invention, based on a branchpredictor predicting a register-indirect branch address, the predictedvalue is also made available as a speculative value in, for instance, anarchitected register. Then, if the prediction is correct, executionproceeds faster. If the prediction is incorrect, recovery is taken, asit would have based on the misprediction.

Further details regarding a prediction technique for a register-indirectbranch are described with reference to FIG. 2. In one example, aprocessor is performing this processing. Initially, the target addressfor a register-indirect branch is predicted, STEP 200, and theinstruction fetch is redirected to the predicted target address (alsoreferred to herein as a predicted address), STEP 202. Further, in oneexample, a new rename register is allocated for the logical registerholding the target address, STEP 204. That is, in one embodiment,register renaming is used for recovery. With register renaming, a newrename register is allocated on a branch, and the new rename register isloaded with the predicted value. The prediction is checked, and if theprediction is incorrect, recovery is performed by returning to thepreviously named register.

As indicated, the predicted address is copied to the allocated renameregister, STEP 206. The rename register holding the target address ismarked as available, STEP 208. Further, a prediction check is performedto determine whether the prediction is correct, STEP 210. There are anumber of ways in which to cause a prediction check to occur, includingstarting a state machine which will perform the check when the targetregister value provided to the branch instruction becomes available;inserting an internal operation (iop) for checking; or causing a branchoperation to be issued to a checking pipeline, etc.

If there is a misprediction, recovery is performed. There are a numberof recovery implementation options that may be used, including copyingthe correct value from the old register to the new register; or using arenaming implementation in which the newly allocated rename register isdiscarded and the rename map points to a previous physical registerholding the correctly loaded value. Further details regarding examplerecovery implementations performed, e.g., by a processor, are describedwith reference to FIGS. 3-4.

Referring initially to FIG. 3, in one embodiment, a check for branchprediction correctness is performed, STEP 300. This includes taking thepredicted address and comparing it to an actual address computed asinput to the branch instruction when it becomes available. If theprediction is correct, INQUIRY 301, then the prediction check iscomplete, STEP 302, and recovery is not needed.

However, if the prediction is incorrect, INQUIRY 301, then instructionsafter the mispredicted branch are flushed, STEP 304. Further, theinstruction fetch is redirected to the computed address, STEP 306.Additionally, the correct non-speculative value is written to theregister holding the target address, STEP 308. There may be a number ofways to accomplish this including in a non-rename design, writing to anactive register. Further, in a rename design, writing to an existingrename register or allocating a new rename register. The registerholding the target address is marked as available, STEP 310. Thiscompletes one example of the prediction check.

In another embodiment, referring to FIG. 4, a prediction check techniquefor a register-indirect branch is provided that explicitly uses registerrenaming. In this example, a check is made of branch predictioncorrectness, STEP 400. If the prediction is correct, INQUIRY 401, thenthe check is complete, STEP 402, and recovery is not performed. However,if the prediction is incorrect, INQUIRY 401, then the instructions afterthe mispredicted branch are flushed, STEP 404, and the rename registerallocated during prediction is deallocated, making the previous renameregister with the non-speculative target address visible, STEP 406. Theinstruction fetch is then redirected to the computed address, STEP 408.This completes one example of the prediction check.

Advantageously, the technique of FIG. 4 corresponds to branch processingas currently implemented in many out-of-order processors, in whichregisters allocated after a branch instruction are automaticallydeallocated, provided the register allocated in conjunction with theprocessing of the branch instruction is treated as a register allocatedafter the execution of the branch instruction. In an embodiment wherethe target register allocated in conjunction with the branch instructionis not treated as allocated after the branch instruction and notdeallocated, an implementation in accordance with FIG. 3 may be adaptedto update the rename register having been allocated to hold thespeculative value with a correct, non-speculative value.

In another embodiment, non-renaming annotations may be used, in which,for instance, two copies of the base are provided. Then, on prediction,a switch is made to the second or new copy. The prediction copies thepredicted value into the new copy, and future instructions use the newlyenabled copy. The prediction is checked, and if there is amisprediction, the old copy is used. In one such implementation,misprediction recovery is performed by updating the active copy of thetarget register, i.e., in which the old (non-predictive) copy is pointedto and the second copy is made active as the next new base value on thenext prediction. Further implementations are also possible.

As described herein, based on predicting contents (e.g., a value; atarget address) of a register to be used for a register-indirect branch,the predicted contents that are used for the prediction are also stored(e.g., concurrently) in the register to be used for theregister-indirect branch so that it may be used by other instructions.This facilitates processing.

In a further aspect, the predicted value that is written into the targetaddress register (and, e.g., the program counter) is also written intoan affiliated register. For instance, if the predicted value is to bewritten into a counter register, CTR, which is the target addressregister, it is also written into another selected register, such asR12, R15, or other register, affiliated with the CTR register.

For certain processing, such as with commonly used ABIs, a branchaddress is first to be loaded into a general purpose register (GPR), andthen, transferred to a special purpose control register (SPR) beforeeffecting a register-indirect branch. In accordance with one suchexample of a branch architecture, the Power ISA branches to a counter(CTR) special purpose register. However, the special purpose register(e.g., the counter register, CTR) is not loaded directly, but is loadedvia a general purpose register.

Since the counter register may be expensive to read out, one example ABIfurther indicates that the value of CTR is to be stored in a selectedregister, e.g., R12, R15, or other register, when a branch to asubroutine (BCTR) is performed. This enables the selected register to beused as a base register by the called function. However, when a branchprediction is made, the branch address may be predicted before the valueof the selected register has been loaded, making the called subroutinestall responsive to a data access.

Previously, to provide the value in the selected register, such as R12in this example, a move from CTR (MFCTR) to R12 was performed that readsthe CTR register and writes the result in R12. However, in accordancewith one definition of an ABI, such as, for example, the Power ELF v2ABI, if a value is made available in CTR, that value is also madeavailable in R12. Thus, the move from CTR to R12 is not needed becausesoftware already indicates that whatever is in the CTR register is alsoto be written in R12, eliminating the copy. However, in accordance withan aspect of the present invention, if a register is predicted, then thepredicted value for that register is made available in that register.Thus, if the predicted register is CTR, CTR is concurrently updated. Theapplication, however, wants to read R12, which has not been updated, andbecause there is no copy, there is no software path to get the predictedvalue that is in CTR into R12. R12 is an affiliated register; it isrelated, but it is not a copy necessarily seen in the code. Therefore,in accordance with an aspect of the present invention, the same valuethat is predicted for CTR and written into CTR is also written into R12(or another selected register or location).

Thus, in accordance with an aspect of the present invention, anaffiliated register is predicted in conjunction with a branch predictionof one or more types of subroutine branches. This is described infurther detail below.

One example of a prediction technique for a register-indirect branchthat also predicts an affiliated register is described with reference toFIG. 5. In one example, a processor is performing this processing.Initially, the target address for a register-indirect branch ispredicted, STEP 500, and the instruction fetch is redirected to thepredicted target address, STEP 502. Further, in one example, a newrename register is allocated for the logical register holding the targetaddress, STEP 504, and the predicted address is copied to the allocatedrename register, STEP 506. The rename register holding the targetaddress is marked as available, STEP 508.

Additionally, in accordance with an aspect of the present invention, anew rename register is also allocated for the affiliated register, STEP510. The predicted address is copied to the rename register allocatedfor the affiliated register, STEP 512, and the rename register for theaffiliated register is marked as available, STEP 514.

A prediction check is performed to determine whether the branchprediction is correct, STEP 516, an example of which is describedherein. Additionally, a prediction check is performed for the affiliatedregister, STEP 518. This check includes, for instance, comparing a valueof the architected value prior to the present instruction beingprocessed responsive to which the affiliated register is being predictedwith the value predicted by this present instruction. The check may beinitiated by starting a state machine, inserting an IOP for checking, orcausing a branch operation to be issued to a checking pipeline with anindication to check the affiliated value. Many variations are possible.

Further details associated with a branch prediction check are describedwith reference to FIG. 6, and further details of a prediction check tocheck the prediction of the affiliated register are described withreference to FIG. 7. In these examples, register renaming is used.However, other implementations are possible without using registerrenaming. In one example, a processor is performing this processing.

Referring initially to FIG. 6, in one example, a check is made of thecorrectness of the branch prediction, STEP 600. If the branch predictionis correct, INQUIRY 601, then the check is complete, STEP 602, andrecovery is not performed. However, if the branch prediction isincorrect, INQUIRY 601, then recovery is performed. For example, theinstructions after the mispredicted branch are flushed, STEP 604. Thismay include the check of the affiliated register. Further, the renameregisters allocated during prediction (e.g., target address register andaffiliated register) are deallocated, making the previous renameregisters with the non-speculative target address and the original valuein the affiliated register visible, STEP 606. The instruction fetch isthen redirected to the computed address, STEP 608. This completes oneimplementation of the branch prediction check.

In one example, the prediction of the affiliated register may be checkedseparate from the check of the branch prediction. Thus, further detailsregarding checking, by e.g., a processor, the prediction of theaffiliated register are described with reference to FIG. 7. In oneexample, correctness of the prediction of the affiliated register ischecked, STEP 700. If the affiliated register prediction is correct,INQUIRY 701, then the check is complete, STEP 702, and recovery is notperformed. However, if the affiliated register prediction is incorrect,INQUIRY 701, then recovery is performed. For instance, the instructionsafter the mispredicted affiliated register (or first user ofmispredicted affiliated register) are flushed, STEP 704, and the renameregister allocated during prediction for the affiliated register isdeallocated, making the previous rename register that includes thenon-predicted correct affiliated value visible, STEP 706. Execution isthen restarted at the flush point, STEP 708. This completes one exampleof the affiliated register prediction check.

In one aspect of checking a prediction, the checking compares thepredicted value of the affiliated register to the most recent value ofthe affiliated register computed prior to the instruction which triggersthe prediction of the affiliated register. In at least one embodiment,that value is stored in a rename register.

In another embodiment, recovery of a misprediction is performed bycopying a value of the affiliated register prior to the instructionwhich triggers the prediction of the affiliated register to the renameregister allocated to hold the predicted value of the affiliatedregister. In at least one embodiment, recovery by restoring a value isused when deallocating a rename register is more expensive than copyingthe correct value to the allocated rename register.

As described herein, based on predicting contents (e.g., a value; atarget address) of a register to be used for a register-indirect branch,the predicted contents that are used for the prediction are also stored(e.g., concurrently) in the register to be used for theregister-indirect branch (in addition to the program counter), as wellas in an affiliated register, so that the value may be used by otherinstructions. This facilitates processing.

In another aspect, a control is provided to specify whether anaffiliated register is to be used in prediction. Further, if it isindicated that the affiliated register is to be used, the control mayinclude an indication of the affiliated register to be used (e.g., aregister number). This control is, for instance, code specific, and maybe enabled/disabled for each unit of code, such as an application, aprocess, a module, a function, or a dynamically shared object (e.g.,library), as examples.

In one aspect, one or more controls are provided for predictingaffiliated registers, in which a first set of units of code benefitsfrom prediction of affiliated register values, and a second set of unitsof code does not. Many variations are possible.

As one example, the control is provided in a machine state register(MSR). The operating system sets the MSR control for one unit of codeand disables the MSR control for another unit of code. In anotherembodiment, a page table entry (PTE) control may be provided. Forexample, a bit in the PTE controls prediction or non-prediction for anaffiliated register for branches on the page where the branch islocated, the page corresponding to a page whose address is translated bythe PTE. In another embodiment, a bit in the PTE controls prediction ornon-prediction of an affiliated register for branches based on thesetting of the bit for the page of the target address of the branch, theaddress of the branch target being translated by the PTE. In yet otherembodiments, hybrid forms are supported, wherein the MSR bit enables,for example, consideration of a bit set in the PTE. Many other examplesare possible.

In one example of a PTE control, a bit in the PTE is set by the dynamicloader based on a module or page level indication of a program module,e.g., describing the module's characteristics, such as ABI level usedfor a particular segment of code.

In at least one embodiment, a static linker links modules so as to groupfunctions into common pages when they use a common ABI level, andseparates them into separate pages when they do not.

In another embodiment, a module shares the ABI level, i.e., the code ina single module corresponds to the same ABI level, and the level andimplied setting for this control is supplied, e.g., by a module header,such as the ELF (Executable Linkable Format) magic number. Many examplesand variations are possible. The example controls described herein arejust examples.

The use of a control to indicate whether affiliated register predictionis to be used for a unit of code is referred to herein, for convenience,as externally indicated affiliated register prediction (EIARP). Oneembodiment of using externally indicated affiliated register predictionfor a unit of code is described with reference to FIG. 8. In oneexample, this processing is performed by an operating system executingon a processor of a computing environment.

Referring to FIG. 8, initially, the operating system loads a unit ofcode, such as an application, process, module, function, or dynamicallyshared object, etc., STEP 800. A determination is made as to whether theunit of code (referred to herein as loaded code or code) is a candidatefor code specific externally indicated affiliated register prediction,STEP 802. This may be determined by, for instance, checking the ABIversion or an EIARP indicator of the code provided in a header or magicnumber of the code, as examples.

If the code is not a candidate for externally indicated affiliatedregister prediction, INQUIRY 804, processing is complete, STEP 810.Otherwise, an indication of the affiliated register (e.g., a registernumber) is loaded into a control register, STEP 806. Example controlregisters include a machine state register (MSR), a program status word(PSW), a special purpose register (SPR), a page table entry (PTE) on apage basis, or a segment table entry (STE), as just a few examples.

Additionally, a control (e.g., a flag or other indicator) may be setthat enables EIARP for the loaded code, STEP 808. In one example, theflag or other indicator is set in a control register or other controlfield, such as in a MSR, PSW, SPR, PTE or STE, as examples. This reducesthe risk of excessive misprediction and recovery penalties.

In another embodiment, a control is set to indicate that a code unit isnot subject to EIARP, when INQUIRY 804 determines that the code is not acandidate for affiliated register prediction, based on the codecharacteristics (e.g., ABI level).

In other embodiments, the code is loaded by one of a program loader, adynamic program loader, a shared library loader, a dynamic libraryloader, or another system component adapted to loading code.

In one embodiment, when there is a context switch of the operatingsystem, the configuration is updated (e.g., by the operating system),including the code specific EIARP. The updating of the code specificEIARP due to a context switch is, e.g., performed when the control islocated in a register that is not specifically bound to the unit ofcode, such as in a MSR, SPR, or PSW. Since the PTE is bound to the code,the EIARP configuration does not need to be updated, in one example. Oneembodiment of context switching logic associated with the code specificEIARP is described with reference to FIG. 9.

Referring to FIG. 9, based on a context switch, a value from the controlregister of the context being switched out is obtained, STEP 900, andstored in a context switch structure, STEP 902. Other processing may beperformed, STEP 904. Additionally, a new value of the context beingswitched in is obtained, STEP 906, and stored into a control register ofthe context being switched in, STEP 908.

In other embodiments, the value for the control register is stored in anapplicant's context structure when the value is initially established,e.g., in conjunction with the processing of the technique of FIG. 8, andSTEPS 900 and 902 to preserve the code's settings are omitted during thecontext switch process. Other variations are also possible.

During processing, a program may take a hardware exception (e.g., pagefault, floating point exception, etc.), and switch to the operatingsystem via interrupt processing. In one embodiment, when there is aswitch to the operating system, the configuration settings may bedisabled (e.g., by the operating system), as described with reference toFIG. 10.

In one example, the value of the control register of the context beingswitched out is obtained, STEP 1000, and stored into an exceptioncontext, STEP 1002. Example exception contexts include, for instance, inlow memory for System z, or select registers for Power Systems, such asSRR0/SRR1 . . . or HSRR0/HSRR1 . . . , as examples. The exceptioncontext may include the code specific EIARP and/or other configurationinformation. Other processing may be performed, STEP 1004. Further,optionally, a supervisor initialization value may be stored in a controlregister, such as in a MSR, SPR, etc., STEP 1006. Additional and/orother processing may also be performed.

Subsequent to handling the exception, hardware exception/interruptionprocessing is exited. One example of this processing, which isperformed, e.g., by a processor, is described with reference to FIG. 11.In one example, the code specific EIARP and/or other configurationinformation to be restored is obtained from the exception context, STEP1100, and stored in a selected control register (e.g., MSR, SPR, PSW,etc.), STEP 1102.

Further details of using a code specific EIARP with branch predictionare described with reference to FIG. 12. In one example, a processor isperforming this processing. In one embodiment, the target address for aregister-indirect branch is predicted, STEP 1200, and the instructionfetch is redirected to the predicted target address, STEP 1202. Further,in one example, a new rename register is allocated for the logicalregister holding the target address, STEP 1204. The predicted address iscopied to the allocated rename register, STEP 1206, and the renameregister holding the target address is marked as available, STEP 1208.

Further, in accordance with an aspect of the present invention, adetermination is made as to whether EIARP is enabled for the code,INQUIRY 1210. This is determined, for instance, by checking a controlindicating whether an affiliated register is to be predicted for thecode. If EIARP is not enabled for the code, then processing continueswith causing a check of the branch prediction to occur, STEP 1220.Otherwise, a new rename register is also allocated for the affiliatedregister, STEP 1212. The register number of the affiliated register isdetermined by, e.g., EIARP configuration information. Further, thepredicted address is copied to the rename register allocated for theaffiliated register, STEP 1214, and the rename register for theaffiliated register is marked as available, STEP 1216.

A check of the prediction of the affiliated register is performed todetermine whether the affiliated register prediction is correct, STEP1218. This includes, for instance, comparing a value of the architectedvalue prior to the present instruction being processed responsive towhich the affiliated register is being predicted with the valuepredicted by this present instruction.

Additionally, a check of the branch prediction is performed, STEP 1220.Examples of check prediction correctness logic are described herein.Examples are also described with reference to FIGS. 14-15 describedbelow. Other examples are also possible.

In yet a further aspect, a prediction is made as to whether anaffiliated register may be predicted. This prediction is based, forinstance, on dynamic predictive determination of affiliation status foran affiliated register candidate (e.g., branch instruction). Aprocessor, in one example, dynamically determines the scope ofaffiliated register for each candidate branch subroutine instructionwhich may have an affiliated register. The branch instructions, thespecific affiliated register candidates and the enablement of thedynamic affiliation determination may be controlled by a variety ofcontrol registers, including an MSR bit, a PTE bit, other controlregisters, or a combination thereof. It may further include a controlregister specifying a candidate for affiliation or a bitmask,determining multiple candidates for affiliation.

In accordance with one or more aspects, a candidate is predicted asaffiliated, and a prediction is made, based on a predictor. A check ofthe prediction is performed in the processor. If the prediction wasincorrect, because the affiliated register is not affiliated, a dynamicpredictor is updated to suppress future affiliated prediction.

There are a plurality of techniques that may be employed to find newdynamic affiliation relationships. One such technique includes, forinstance, a random guess of affiliation when a predictor indicatesnon-affiliation (at a sufficiently low rate), to identify either actualchanged behavior or changed behavior due to aliasing where anotherbranch becomes dominant. Another technique includes, for example,training periods in which the operating system determines to force newaffiliations to be predicted, either at a fixed interval, or responsiveto provided changes, e.g., as determined with a variety of dynamicruntime monitoring and fingerprint taking techniques. Other techniquesare also possible.

In one aspect, a prediction is made as to whether a defined register(either statically defined or defined in an EIARP configuration, asexamples) is affiliated for a specific instance (e.g., a specificbranch). In this aspect, a unit of code (e.g., a branch instruction) isloaded, and a defined register number (e.g., R12) is hardwired in thehardware or loaded in a configuration or control register (e.g., MSR,PSW, SPR, PTE, STE, etc.). Optionally, a flag to enable prediction forthe unit of code is also set.

If the register number is loaded in a configuration or control register,and not hardwired, processing is performed, based on a context switch,since each unit of code may have different configuration information. Anexample of this processing is described with reference to FIG. 9.

Similarly, if a hardware exception is taken and the register number isnot hardwired, exception processing may be performed, as described, inone embodiment, with reference to FIGS. 10 and 11.

One example of a prediction technique for a register-indirect branchthat also predicts whether a defined register is affiliated is describedwith reference to FIG. 13. In one example, a processor is performingthis processing. In one example, initially, the target address for aregister-indirect branch is predicted, STEP 1300, and the instructionfetch is redirected to the predicted target address, STEP 1302. Further,in one example, a new rename register is allocated for the logicalregister holding the target address, STEP 1304. The predicted address iscopied to the allocated rename register, STEP 1306, and the renameregister holding the target address is marked as available, STEP 1308.

Further, a determination is made as to whether the affiliated value maybe predicted, INQUIRY 1310. For instance, may the affiliated register bepredicted? If not, then processing continues with causing a branchprediction check to occur, STEP 1320. Otherwise, in one embodiment, anew rename register is allocated for the affiliated register beingpredicted, STEP 1312. The register number of the affiliated register maybe hardwired or determined by EIARP configuration information, asexamples.

The predicted address is copied to the allocated rename register for theaffiliated register, STEP 1314, and the rename register for theaffiliated register is marked as available, STEP 1316.

A check of the prediction of the affiliated register is performed todetermine whether the prediction is correct, STEP 1318, as well as acheck of the branch prediction, STEP 1320, as described herein. If thereis a misprediction, recovery is performed, as described with referenceto FIGS. 14-15. In one example, a processor is performing thisprocessing.

In one or more embodiments, INQUIRY 1310 may be subject to a controloverride to discover new affiliated registers. In one embodiment,INQUIRY 1310 is forced to indicate an affiliated register during atraining phase. A training phase may correspond, for example, to a fixedor configurable number of cycles after an application has been loaded.In another example, a training phase may correspond, for example, to afixed or configurable number of cycles after an indication to start atraining phase. In one example, a start of training indication maycorrespond to one of a randomly generated signal at a fixed orconfigurable probability. In another example, a start of trainingindication may correspond to the passing of a specified interval sincethe last training phase, e.g., a fixed or configurable number of cycles,or a fixed or configurable number of nanoseconds, microseconds, oranother time unit. In another embodiment, INQUIRY 1310 indicates thepresence of an affiliated register for a first number of executions of abranch; the first number being fixed or configurable. Yet otherembodiments may employ other known or future techniques to train anaffiliated register predictor.

Referring now to FIG. 14, in this example, by definition, if there is abranch misprediction, then prediction of or relating to the affiliatedvalue is also mispredicted. The check includes, for instance,determining the correctness of the branch prediction, STEP 1400. If thebranch prediction is correct, INQUIRY 1401, then the check is complete,STEP 1402, and recovery is not performed. However, if the branchprediction is incorrect, INQUIRY 1401, then recovery is performed. Forinstance, the instructions after the mispredicted branch are flushed,STEP 1404. This may include flushing the affiliated register check.Further, the rename registers allocated during prediction aredeallocated, making the previous rename registers visible, STEP 1406.This may include deallocating the target address register, as well asthe affiliated register, if EIARP is enabled, making the previous renameregisters with the non-speculative target address and the original valuein the affiliated register visible. The instruction fetch is thenredirected to the computed address, STEP 1408. This completes oneimplementation of the prediction check.

Additionally, in one embodiment, if EIARP is enabled, a check may beinvoked for correctness of the affiliated register prediction, asdescribed with reference to FIG. 15. In one embodiment, a check is madeof affiliated register prediction correctness, STEP 1500. If theprediction of the affiliated register is correct, INQUIRY 1501, then thecheck is complete, STEP 1502, and recovery is not performed. However, ifthe prediction of the affiliated register is incorrect, INQUIRY 1501,then recovery is performed. For instance, the instructions after themispredicted affiliated register are flushed, STEP 1504. In anotherexample, the first user of the mispredicted affiliated register isflushed at STEP 1504. Additionally, the rename register allocated duringprediction for the affiliated register is deallocated, making theprevious rename register that includes the non-predicted correctaffiliated value visible, STEP 1506. Execution is then restarted at theflush point, STEP 1508. This completes one example of the affiliatedregister prediction check.

In yet a further embodiment, if a predictor is used to predict whetheran affiliated value (e.g., affiliated register) may be predicted, then apredictor update associated with this prediction may also be performed,as described with reference to FIG. 16. In one example, a processor isperforming this processing.

Referring to FIG. 16, in one embodiment, a check is made of affiliatedregister prediction correctness, STEP 1600. If the prediction of whetherthe affiliated register may be predicted is correct, INQUIRY 1601, thenthe affiliation predictor may be updated to indicate a correctprediction, STEP 1602, and recovery is not performed. In anotherexample, no update need be performed.

However, if the prediction of whether the affiliated register may bepredicted is incorrect, INQUIRY 1601, then recovery is performed. Forinstance, the instructions after the mispredicted affiliated registerare flushed, STEP 1604. In another embodiment, the first user of amispredicted affiliated register is flushed at STEP 1604. Additionally,the rename register allocated during prediction for the affiliatedregister is deallocated, making the previous rename register thatincludes the non-predicted correct affiliated value visible, STEP 1606.Further, in one example, the affiliation predictor is updated toindicate an incorrect prediction, STEP 1608. Execution is then restartedat the flush point, STEP 1610. This completes one implementation of theprediction check.

In an alternate embodiment, the value stored in the incorrectlypredicted affiliated register is copied into the rename register in lieuof deallocating the rename register. Other variations are possible.

In a further aspect, a prediction is made as to whether a register isaffiliated for a specific instance and a further prediction is made asto the register number or other identification. One example of thisprocessing is described with reference to FIG. 17. In one example, aprocessor is performing this processing.

Referring to FIG. 17, initially, the target address for aregister-indirect branch is predicted, STEP 1700, and the instructionfetch is redirected to the predicted target address, STEP 1702. Further,in one example, a new rename register is allocated for the logicalregister holding the target address, STEP 1704. The predicted address iscopied to the allocated rename register, STEP 1706, and the renameregister holding the target address is marked as available, STEP 1708.

Further, in accordance with an aspect, a determination is made as towhether the affiliated value may be predicted, INQUIRY 1710. If not,then processing continues with causing a branch prediction check tooccur, STEP 1726. Otherwise, a determination is made as to whether a newregister identification (e.g., number) is to be selected as thepredicted affiliated register, INQUIRY 1712. If not, then an indicationof a predicted affiliated register indicated by, e.g., a predictor isselected as the affiliated register, STEP 1714; otherwise, anotherregister (e.g., other register number) is selected, STEP 1716.Thereafter, a new rename register is allocated for the selectedaffiliated register (e.g., having the selected register number), STEP1718.

The predicted address is copied to the allocated rename register for theaffiliated register, STEP 1720, and the rename register for theaffiliated register is marked as available, STEP 1722.

A check of the prediction of the affiliated register is performed todetermine whether prediction associated with the affiliated register iscorrect, STEP 1724, as well as a check of the branch prediction, STEP1726, as described herein.

In at least one embodiment, INQUIRY 1712 corresponds to a test ofwhether a prediction confidence exceeds a threshold value. Predictionconfidence can correspond, for example, to a known confidence value of 1or more bits. The threshold may be fixed or software configured.

Prediction checking may also be performed to determine whether theselected affiliated register is correct, as described with reference toFIG. 18. In one example, a processor is performing this processing. Inone embodiment, a check is made of affiliated register predictioncorrectness, STEP 1800. If the prediction of the selected affiliatedregister is correct, INQUIRY 1801, then the affiliation predictor may beupdated to indicate a correct prediction of the register being used asthe affiliated register, STEP 1802, and recovery is not performed. Inanother embodiment, the predictor is not updated.

However, if the prediction of the selected affiliated register isincorrect, INQUIRY 1801, then recovery is performed. For instance, theinstructions after the mispredicted affiliated register are flushed,STEP 1804. In another embodiment, the first user of the mispredictedaffiliated register is flushed at STEP 1804. Additionally, the renameregister allocated during prediction for the affiliated register isdeallocated, making the previous rename register that includes thenon-predicted correct affiliated value visible, STEP 1806. Further, inone example, the affiliation predictor is updated to indicate anincorrect prediction of the specific register to be used as theaffiliated register, STEP 1808. Execution is then restarted at the flushpoint, STEP 1810. This completes one implementation of the affiliatedregister prediction check.

In accordance with a further aspect, affiliated registers are recognizedby recognizing a sequence of instructions. In one example, the sequenceincludes a first instruction adapted to create an affiliatedrelationship, and a second instruction adapted to perform a subroutinebranch. In one example, the sequence includes:

mtctr R12 bctrl

This sequence is recognized as performing both the affiliation-creatingmove and the subroutine call. In one embodiment, theaffiliation-creating sequence is translated into a fusion sequence ofoperations that performs the move, and creates a branch with anaffiliation prediction.

Based on recognizing the above sequence (or a similar sequence), afusion sequence of operations is generated that performs the move andthe branch. As examples, a simplified sequence may be generated, e.g.,with R12 hardwired or with EIARP; or an enhanced fusion sequence may begenerated that includes a check of the predicted branch address,obviating the cost of the MTCTR instruction altogether. One example ofan enhanced sequence is described with reference to FIG. 19. In oneembodiment, this processing is performed by a decoder.

Initially, a determination is made as to whether a fusion sequenceopportunity has been recognized, INQUIRY 1900. For instance, has thedecoder recognized MTCTR followed by BCTRL, as an example? If a fusionsequence opportunity has not been recognized, fusion generationprocessing is complete. Otherwise, fusion generation processingproceeds. A branch target address is predicted, STEP 1902. The targetaddress is loaded into a program counter (PC), such that processing canbegin executing at the target address, STEP 1904. Further, the predictedtarget address is loaded into the CTR register, STEP 1906, as well asinto an affiliated register (e.g., R12), STEP 1908. Since this is asubroutine call, a return address is loaded into a link register (LR),STEP 1910. Additionally, a check of the predicted target address isperformed, STEP 1912. In one example, this check is against the sourceof MTCTR, which is the value of R12.

In one example, the affiliated register number (e.g., R12) is hardwiredas the affiliated register. In another example, the affiliated registeris indicated in a control, such as in a selected control register. Inyet a further example, it is dynamically determined based on thesequence of instructions (e.g., register number specified after MTCTR).Other examples are also possible. With the above sequence, there is noneed to execute MTCTR, since the prediction overwrites the value, andthe affiliated register and predicted target address are checkedsimultaneously, in one example. Other variations are possible.

In one example embodiment, the operations of the technique of FIG. 19may be expressed by generating an example iop sequence, such as:

mtctr R12 old ctr=ctr update_lr_from_instruction_address+predict_indirect_ctr+ update _pc_from_prediction+update_ctr_from_prediction+ update_affiliated_reg_from_prediction (R12)check_target_address (old_ctr)

In another embodiment, the copy of the R12 is suppressed, and asimplified iop sequence is generated:

old_affiliated=R12+ update_lr_from_instruction_address+predict_indirect_ctr+ update _pc_from_prediction+update_ctr_from_prediction+ update_affiliated_reg_from_prediciton (R12)check_target_address (old_affiliated)

In a variety of embodiments, the prediction iop generates a singleprediction which may be used to update the program counter (i.e., PC—theaddress from which instructions are fetched), the branch target register(ctr in this example), and an affiliated register (R12 in this example).In this particular embodiment, the prediction iop is also adapted tocapture the PC prior to updating with the prediction in order to capturea subroutine return address, e.g., in the link register (LR). In anotherembodiment, the capturing of the PC in the link register (LR) may beperformed by a separate iop.

In these examples, the + is used to indicate the performance of multipleoperations in a single iop, the descriptive components connected bythe + sign. For instance, update_pc_from_prediction updates the programcounter (PC) with the address predicted by the predictor.

In one embodiment with an externally indicated affiliated register,recognition of a fusion sequence is based on recognizing a sequenceincluding a register indicated in a configuration register. Prior todetection of a fusion-based affiliated sequence, an affiliated registeris indicated using a control register for external indication of theaffiliated register sequence to be recognized. One example of thisprocessing, which is performed, e.g., by a processor, is described withreference to FIG. 20. In another embodiment, the sequence establishingthe affiliated register relationship and including the affiliatedregister number is fixed (or “hardwired”) in the processor logic, andthe technique of FIG. 20 is not performed in such an embodiment.

Referring to FIG. 20, initially, the code (such as code containing oneor more fusible sequences establishing an affiliated registerrelationship) is loaded, STEP 2000. Further, a register numbercorresponding to a register which may occur in an affiliation-generatingsequence to be recognized is loaded, STEP 2010. Further, optionally, aflag is set to enable EIARP, STEP 2020. Since only sequences that arerecognized as establishing affiliated register relationships aretranslated, there is no risk of excessive mispredictions and hence afacility to enable or disable affiliated register prediction may not bepresent in one or more embodiments. In other embodiments, a flag forenabling or disabling the prediction of the value of an affiliatedregister may be provided as a safety feature, e.g., in case of anincorrect implementation of the affiliated register prediction logic.Other variations are also possible. Other variations are also possible.

If the register to be used as an affiliated register is not hardwired,but instead, indicated in a control, then the configuration information,including an indication of the affiliated register, is stored for laterrestoration, based on a context switch. One embodiment of thisprocessing is described above with reference to FIG. 9. Similarly, if ahardware exception is taken, processing is performed, as described withreference to FIGS. 10-11. However, if the register number is hardwired,this processing is optional.

One embodiment of branch prediction with a fusion-based affiliatedsequence is described with reference to FIG. 21. In one example, aprocessor is performing this processing.

Referring to FIG. 21, a fusion opportunity detection step is performed,STEP 2100. For instance, a check is made by the decoder for a sequenceof MTCTR Rx/BCTRL, where Rx corresponds to a register for which anaffiliation relationship with the branch target address may be detected.In one example sequence, register Rx may correspond to register R12.Based on performing a detection step for an affiliation-creatingsequence which may be fused, a determination is made as to whether sucha fusion sequence opportunity is detected, INQUIRY 2102. That is, is asequence of MTCTR Rx and BCTRL present in the code stream, and does Rxof MTCTR match an expected register number, such as a hardwiredaffiliated register number, or a number provided in a control, e.g.,EIARP control, as examples? If a fusion sequence opportunity is notdetected, conventional processing is performed, STEP 2104. Otherwise,processing continues with predicting the target address for aregister-indirect branch, STEP 2106, and redirecting the instructionfetch to the predicted target address, STEP 2108. Further, in oneexample, a new rename register is allocated for the logical registerholding the target address, STEP 2110. The predicted address is copiedto the allocated rename register, STEP 2112, and the rename registerholding the target address is marked as available, STEP 2114.

Additionally, a new rename register is allocated for the affiliatedregister (e.g., the register Rx specified in the affiliation-creatingfusion sequence), STEP 2116. The predicted address is copied to therename register allocated for the affiliated register, STEP 2118, andthe rename register for the affiliated register is marked as available,STEP 2120.

A check of the prediction of the predicted address is performed todetermine whether the prediction is correct, STEP 2122. This includes,for instance, comparing the target address to a value of the affiliatedregister (Rx) prior to the fusion sequence (e.g., using its previousrename register). If the prediction is incorrect, then recovery isperformed, as described herein. One example of such recovery isdescribed with reference to FIG. 14.

In further embodiments, the affiliated register number is determined bythe sequence itself (e.g., register number specified after MTCTR),rather than hardwired or provided by a control (i.e., EIARP). In onesuch example embodiment, INQUIRY 2102 checks whether there is a sequenceof MTCTR RX and BCTRL present in the code stream. If the sequence ispresent, then the particular register RX specified in the sequence isused as the affiliated register. In one such embodiment, the codeloading and EIARP configuration technique, such as that of FIG. 8 orFIG. 20, are not necessary, and affiliated register prediction mayoperate in conjunction with conventional code loading techniques.Further, saving the affiliated register information on a context switchand/or performing hardware exception processing is not needed, andaffiliated register prediction may operate in conjunction withconventional context switch sequences and hardware exception processing,in at least one embodiment.

In the above example of a sequence of instructions, the MTCTR and BCTRLinstructions are in sequential order. However, in accordance with afurther aspect, affiliation is predicted even if the instructions arenot in sequential order. For instance, assume the following code:

mtctr R12 <instructions...> bctrl

The above code is recognized as performing both the affiliation-creatingmove and the subroutine call. Thus, based on recognizing an instructionthat creates an affiliation (e.g., MTCTR), an indicator is set. Theindicator may further specify the number (or other indication) of theaffiliated register. Then, when an operation destroying the affiliationis recognized (e.g., either changing the contents of CTR, or theregister that was the source of MTCTR), the indicator is reset. When aBCTRL is encountered, it is indicated as a branch with an affiliatedregister if an affiliation indication is active, further optionallyindicating the register number being affiliated.

One embodiment of processing associated with determining affiliationbased on dynamic runtime control analysis is described with reference toFIG. 22. In one example, a processor is performing this processing. Thetechnique of FIG. 22 is performed on each instruction of the instructionstream sequence to identify affiliation of a register to a branch targetregister of a branch, identify branches for which an affiliated registershould be predicted, as well as the register number of the affiliatedregister. Detected affiliation relationships are passed from theexecution of the technique of FIG. 22 for one instruction to theexecution of this technique for the next instruction, until anaffiliation is either cancelled, or a register-indirect branch isdetected in conjunction with which the prediction of the value of anaffiliated register is to be performed. This processing is performed bythe instruction decoder, in one example.

Referring to FIG. 22, in one embodiment, an instruction is received,STEP 2200, and decoded, STEP 2202. A determination is made as to whetherthe instruction is an affiliation-creating instruction, such as anMTCTR, INQUIRY 2204. If it is an affiliation-creating instruction, anyprevious affiliation is cancelled, and the new affiliation is recorded(e.g., an indicator is set), STEP 2206. However, if the instruction isnot an affiliation creating instruction, then a further determination ismade as to whether the instruction is an affiliation-destroyinginstruction (e.g., an instruction that overwrites either the source orthe target of the affiliation relationship, e.g., either the sourceregister of MTCTR which established the affiliation, or the countregister, CTR, respectively), INQUIRY 2208. If it is an affiliationdestroying instruction, then the affiliation is cancelled (e.g., theindicator is reset), STEP 2210.

If the instruction is not an affiliation-creating or anaffiliation-destroying instruction, then this processing continues withINQUIRY 2212, which tests whether the present instruction is aregister-indirect branch instruction, and an affiliation is currentlyactive. If INQUIRY 2212 is positive, then a register-indirect branchinstruction with an affiliated register has been detected, and controlpasses to STEP 2214. Otherwise, the technique ends, for the presentinstruction.

At STEP 2214, a register-indirect branch instruction has been detectedwhen an affiliation relationship between a register-indirect branchtarget register (such as the counter register, ctr) and another registeris active. In order to facilitate the prediction of the value of thedetected affiliated register, the presence of an affiliated register isindicted in conjunction with the branch instruction, and the registernumber of the affiliated register is recorded, so as to facilitate theprediction of the value of the affiliated register in conjunction withbranch processing for the present branch. The technique of FIG. 22terminates for the present instruction.

When the technique of FIG. 22 completes for one instruction beingprocessed for a processor (e.g., after STEPS 2206, 2210, 2212 or 2214),the processing is performed on the next fetched instruction. Therecorded affiliation information is passed from one execution of thetechnique of FIG. 22 to the next execution for the next instruction inthe instruction stream.

Based on determining the sequence of an affiliation-creating move and asubsequent branch in accordance with STEP 2214, even though they may beseparated by one or more other instructions, a fusion sequence isgenerated, based on the affiliation-creating move and the branch, asdescribed above. As an example, the processing of FIG. 19, as well asFIGS. 20-21, may be performed, in addition to recovery, as describedwith reference to FIG. 14. Other processing described with fusion and/orpredictive branching may also be performed in one or more embodiments.

As described herein, in one embodiment, to avoid the cost of performinga move from counter instruction, MFCTR, (which moves the value from CTRto a selected register (e.g., R12)), when the CTR value is predicted, anABI may specify the presence of the value of the CTR in a generalregister (e.g., register R12, in accordance with the known ELFv2 ABI ofthe Power Architecture), and, the predicted value is written into CTRand the selected register (e.g., R12) by a processor, when a sequencecorresponding to the ABI function call sequence using an affiliatedregister (such as R12) to transfer the called function's address to thecalled function is detected in accordance with an aspect of the presentinvention.

This selected register is referred to herein as an affiliated register.In one example, the value of the selected register is used to determineanother value, such as a pointer to a table of contents (TOC) or aglobal offset table (GOT), in accordance with commonly known ABIspecifications, such as the ELF v2 ABI of the Power Architecture. Thisother value is stored in another selected register, such as R2. Forinstance, the value of the other selected register, R2, is obtained byadding an offset to the value in the selected register (e.g., R12). Thevalue of the other selected register (e.g., R2) is then used, forinstance, as a pointer to the TOC or GOT, which provides access tovariables. In particular, a compiler generates object code from sourcecode, without knowing the final address or displacement of thecode/data. Specifically, the compiler generates object code that willaccess a variable address reference data structure (e.g., a GlobalOffset Table or a Table of Contents (TOC)) for variable values withoutknowing the final size of the data structure or offsets/addresses ofvarious data sections. Placeholders for this information are left in theobject code and updated by a linker.

Since the value of the affiliated register (e.g., R12) is beingpredicted and the offset is known or determinable, in accordance with anaspect, the value of the other selected register is predicted as anaffiliated derived value.

In one implementation, the configuration and processing of theaffiliated derived value is similar to the configuration and processingof the affiliated value. For instance, an indication of the affiliatedderived value (e.g., register number, such as R2) may be hardwired orstored in a control register or other control field, such as an MSR,SPR, PSW, PTE, STE, etc. Further, if the code is a candidate for EIARP,then the register number of the affiliated derived register may beloaded (e.g., STEP 806 of FIG. 8). As further examples, the value may bestored and restored in a context switch, as described in one examplewith reference to FIG. 9, and be included in exception processing, asdescribed in one example with reference to FIGS. 10-11. It may also beincluded in other affiliated register and/or register-indirect branchprediction processing, described herein.

One example of a prediction technique for a register-indirect branchthat also predicts an affiliated derived register is described withreference to FIG. 23. In one example, a processor is performing thisprocessing. Initially, processing associated with a register-indirectbranch and affiliated value prediction is performed, STEP 2300. This mayinclude a number of operations, including, for instance: predicting thetarget address for a register-indirect branch; redirecting theinstruction fetch to the predicted target address; allocating a newrename register for the logical register holding the target address;copying the predicted address to the allocated rename register; markingthe rename register holding the target address as available; allocatinga new rename register for the affiliated register; copying the predictedaddress to the rename register allocated for the affiliated register;marking the rename register for the affiliated register as available;and performing one or more prediction checks, as described, in oneexample, with reference to FIG. 5.

In addition to the above processing, in which one or more of the aboveoperations are performed, a determination is made as to whether thepredictor predicts an affiliated derived register, INQUIRY 2302. In oneexample, this is based on recognizing a sequence of instructions thatrepresents use of a GOT. In a further example, one or more controls in acontrol register may be used to predict whether the value of a derivedaffiliated register to be predicted. Other examples are also possible.

If the predictor does not predict an affiliated derived register, thenthis processing is complete. In one example, when the code branches to asequence computing a derived value, dynamic idiom recognition isperformed during execution. A sequence computing a derived value isrecognized, e.g., dynamically during the execution of the code, and apredictor is updated to indicate such.

Otherwise, if the predictor does predict an affiliated derived value, inone example, an offset for the derived value is obtained, STEP 2304.This offset may be obtained using a variety of techniques, includingusing a look-up table, accessing the instruction stream or using apredictor, as examples. Many techniques are possible. Further, a valuefor the affiliated derived register is computed by, for instance, addingthe offset to a predicted value of the affiliated register obtained inSTEP 2300, STEP 2306.

A new rename register is allocated for the affiliated derived register,STEP 2308. The computed value is copied to the allocated rename registerfor the affiliated derived register, STEP 2310, and the rename registerfor the affiliated derived register is marked as available, STEP 2312.

A check of the prediction of the affiliated derived register may beperformed to determine whether the prediction is correct, STEP 2314.Further details associated with a prediction check that includeschecking the prediction of the affiliated derived register are describedwith reference to FIG. 24. This may be performed in addition to thebranch prediction check and/or the affiliated register predictionchecks, as examples. In one example, a processor is performing thisprocessing.

Referring to FIG. 24, in one example, a check is made of the correctnessof the affiliated derived prediction, STEP 2400. If the prediction iscorrect, INQUIRY 2401, then the check is complete, STEP 2402, andrecovery is not performed. However, if the prediction is incorrect,INQUIRY 2401, then recovery is performed. For instance, the instructionsafter the function entry are flushed, STEP 2404. Further, the renameregisters allocated during prediction are deallocated, including therename register for the affiliated derived value, STEP 2406.Additionally, the instruction fetch is then redirected to re-execute thecomputations of the affiliated derived register (e.g., R2) andsubsequent instructions, STEP 2408. This completes one implementation ofthe prediction check.

In other implementations, the indication of the affiliated derivedregister (e.g., register number) may be predicted. Thus, processing isperformed, as described herein, for the affiliated register, to predictthe indication, use the prediction, check the correctness, e.g., bycomparing the predicted value of the affiliated derived register withthe value computed by the instruction sequence computing the valuenon-predictively in the instruction stream and/or recover therefrom.Many variations are possible.

In a further aspect, the presence of an affiliated derived register andthe particular register number of the register may be determinedresponsive to detecting one or more sequences of instructions. Forinstance, the decode logic recognizes the following code sequence:

mtctr Rx bctrl addis Ry, Rx, #higha (_foo - .TOC.) addi Ry, Ry, #low(_foo - .TOC.)

In the above code, addis is add immediate shift, e.g., in accordancewith the definition of the Power Architecture in one example embodiment;addi is add immediate, e.g., in accordance with the definition of thePower Architecture in one example embodiment; Rx is the affiliatedregister (e.g., R12); and Ry is the affiliated derived register (e.g.,R2).

Based on recognizing the above sequence (or a similar sequence;different sequences are possible and a hardware implementation mayrecognize one or more of the possible sequences, in accordance with avariety of example embodiments), a fusion sequence is generated thatperforms the move, branch and computing the derived value. As examples,a simplified fusion sequence may be generated, e.g., with the affiliatedregister (e.g., R12) and the affiliated derived register (e.g., R2)hardwired or specified in a control register; or an enhanced fusionsequence may be generated that includes a check of the predicted branchaddress against the original affiliated register, obviating the cost ofthe MTCTR altogether. In one embodiment, this processing is performed bya decoder and includes, for instance:

predict target address PC <= predicted target address CTR <= predictedtarget address Rx <= predicted target address Ry <= predicted targetaddress + offset return address check predicted target address againstRx

In one example, the affiliated register number (e.g., R12) and/or theaffiliated derived register number (e.g., R2) are hardwired as theaffiliated register and the affiliated derived register, respectively.In another example, the affiliated register and/or the affiliatedderived register number is indicated in a control, such as in a selectedcontrol register. In yet a further example, they are dynamicallydetermined based on the sequence of instructions (e.g., register numberspecified after MTCTR or the register number provided in the addis oraddi). Other examples are also possible.

Further details regarding a prediction technique for a register-indirectbranch with a fusion-based affiliated derived sequence are describedwith reference to FIG. 25. In one example, a processor is performingthis processing. In one embodiment, a fusion opportunity detection stepis performed, STEP 2500. For instance, a check is made by the decoderfor a sequence of MTCTR Rx/BCTRL./ADDIS/ADDI. Based on performing thedetecting step, a determination is made as to whether such a fusionsequence opportunity is detected, INQUIRY 2502. That is, is a sequenceof MTCTR, BCTRL, ADDIS and ADDI present in the code stream, and does Rxof MTCTR match an expected register number, such as a hardwiredaffiliated register number, or a number provided in a control, asexamples? If a fusion sequence opportunity is not detected, conventionalprocessing is performed, STEP 2504. Otherwise, processing continues, inone example, with predicting the target address for a register-indirectbranch, STEP 2506, and redirecting the instruction fetch to thepredicted target address, STEP 2508. Further, in one example, a newrename register is allocated for the logical register holding the targetaddress, STEP 2510. The predicted address is copied to the allocatedrename register, STEP 2512, and the rename register holding the targetaddress is marked as available, STEP 2514.

Further, a new rename register is also allocated for the affiliatedregister (e.g., Rx, such as R12), STEP 2516. The predicted address iscopied to the rename register allocated for the affiliated register,STEP 2518, and the rename register for the affiliated register is markedas available, STEP 2520.

Additionally, a new rename register is also allocated for the affiliatedderived register (e.g., Ry, such as R2), STEP 2522. An offset is addedto the predicted address and the result is stored in the rename registerallocated for the affiliated derived register, STEP 2524. The renameregister for the affiliated derived register is marked as available,STEP 2526.

A check of the prediction of the predicted address is performed todetermine whether the prediction is correct, STEP 2528. This includes,for instance, comparing the target address to a value of the affiliatedregister (Rx) prior to the fusion sequence (e.g., using its previousrename register). If the prediction is incorrect, then recovery isperformed, as described herein, in one example. This completes oneembodiment of this processing.

In other embodiments, other prediction checks may also be performed, asdescribed herein.

One or more aspects of the present invention are inextricably tied tocomputer technology and facilitate processing within a computer,improving performance thereof. Further details of one embodiment offacilitating processing within a computing environment, as it relates toone or more aspects of the present invention, are described withreference to FIGS. 26A-26B.

Referring to FIG. 26A, in one embodiment, a predicted value to be usedin register-indirect branching is predicted using a processor of thecomputing environment (2600). The predicted value is stored in aselected location (e.g., a hardware register) accessible to one or moreinstructions of the computing environment (2602). The storing isperformed concurrently to processing of a register-indirect branch(2604). Further, the selected location is in addition to anotherlocation (e.g., PC) used to store an instruction address (2605). Thepredicted value is used in speculative processing that includes theregister-indirect branch (2606). As an example, the predicted valueincludes a target address to be used by an instruction fetch (2608), andthe using includes redirecting the instruction fetch to the targetaddress (2610).

In a further aspect, a determination is made as to whether the predictedvalue is accurate (2612). Based on determining the predicted value isinaccurate, recovery is performed (2614). Recovery may be performed anumber of ways. In one example, the recovery includes, for instance,flushing one or more instructions executed subsequent to the using(2616), and replacing the predicted value with a non-predicted value(2618). As one example, the replacing includes using one or more renameregisters to provide the non-predicted value (2620).

Further, in one example, referring to FIG. 26B, the storing includesallocating a register to hold the predicted value (2622), and copyingthe predicted value to the register allocated to hold the predictedvalue (2624).

In one particular example, the register is a rename register allocatedfor a logical register assigned to hold the predicted value (2626).Additionally, in one example, the rename register is marked as available(2628).

One or more aspects of the present invention are advantageously used toaccelerate instruction sequences. In accordance with one embodiment forthe Power architecture, the sequence provided below can be executedefficiently, in which the counter register is used to initialize a GOTbase when the function is entered via a counter-indirect branch.

Thus, a caller may call a subroutine, as indicated below, whenperforming a register indirect or a module external call, and when thecounter (CTR) register has been initialized with the target register ofthe function to be called:

bctrl

In accordance with one embodiment, an ABI defines a function to have twoentry points, one for count register (ctr)-indirect calls from moduleexternal callers and register-indirect calls when a GOT pointer is to beinitialized, and from a local caller when a GOT pointer is not to beinitialized:

callee_ctr_indirect_entry: mfctr r12 ! obtain CTR value addis r2, r12,(.TOC.-callee_ctr_indirec_entry)@ha addi r2.r2,(.TOC.-callee_ctr_indirect_entry)@1 calle_direct_local_entry: ... !subroutine body blr ! return to caller

Advantageously, the MFCTR instruction loading the value of the countregister can provide the predicted speculative value of CTR immediately,enabling the subsequent sequence of addis and addi to compute the GOTpointer as a register to access global variables, and enabling globalvariable accesses to proceed that would otherwise delay the processingof the subroutine body.

In another embodiment, e.g., in accordance with the System/360instruction set, subroutine calls may be performed using a registerindirect call, and only a single entry point may be present.

Other types of computing environments may also incorporate and use oneor more aspects of the present invention, including, but not limited to,emulation environments, an example of which is described with referenceto FIG. 27A. In this example, a computing environment 20 includes, forinstance, a native central processing unit (CPU) 22, a memory 24, andone or more input/output devices and/or interfaces 26 coupled to oneanother via, for example, one or more buses 28 and/or other connections.As examples, computing environment 20 may include a PowerPC processor ora Power server offered by International Business Machines Corporation,Armonk, N.Y.; and/or other machines based on architectures offered byInternational Business Machines Corporation, Intel, or other companies.

Native central processing unit 22 includes one or more native registers30, such as one or more general purpose registers and/or one or morespecial purpose registers used during processing within the environment.These registers include information that represents the state of theenvironment at any particular point in time.

Moreover, native central processing unit 22 executes instructions andcode that are stored in memory 24. In one particular example, thecentral processing unit executes emulator code 32 stored in memory 24.This code enables the computing environment configured in onearchitecture to emulate another architecture. For instance, emulatorcode 32 allows machines based on architectures other than thez/Architecture, such as PowerPC processors, pSeries servers, or otherservers or processors, to emulate the z/Architecture and to executesoftware and instructions developed based on the z/Architecture.

Further details relating to emulator code 32 are described withreference to FIG. 27B. Guest instructions 40 stored in memory 24comprise software instructions (e.g., correlating to machineinstructions) that were developed to be executed in an architectureother than that of native CPU 22. For example, guest instructions 40 mayhave been designed to execute on a z/Architecture processor, butinstead, are being emulated on native CPU 22, which may be, for example,an Intel processor. In one example, emulator code 32 includes aninstruction fetching routine 42 to obtain one or more guest instructions40 from memory 24, and to optionally provide local buffering for theinstructions obtained. It also includes an instruction translationroutine 44 to determine the type of guest instruction that has beenobtained and to translate the guest instruction into one or morecorresponding native instructions 46. This translation includes, forinstance, identifying the function to be performed by the guestinstruction and choosing the native instruction(s) to perform thatfunction.

Further, emulator code 32 includes an emulation control routine 48 tocause the native instructions to be executed. Emulation control routine48 may cause native CPU 22 to execute a routine of native instructionsthat emulate one or more previously obtained guest instructions and, atthe conclusion of such execution, return control to the instructionfetch routine to emulate the obtaining of the next guest instruction ora group of guest instructions. Execution of native instructions 46 mayinclude loading data into a register from memory 24; storing data backto memory from a register; or performing some type of arithmetic orlogic operation, as determined by the translation routine.

Each routine is, for instance, implemented in software, which is storedin memory and executed by native central processing unit 22. In otherexamples, one or more of the routines or operations are implemented infirmware, hardware, software or some combination thereof. The registersof the emulated processor may be emulated using registers 30 of thenative CPU or by using locations in memory 24. In embodiments, guestinstructions 40, native instructions 46 and emulator code 32 may residein the same memory or may be disbursed among different memory devices.

As used herein, firmware includes, e.g., the microcode or Millicode ofthe processor. It includes, for instance, the hardware-levelinstructions and/or data structures used in implementation of higherlevel machine code. In one embodiment, it includes, for instance,proprietary code that is typically delivered as microcode that includestrusted software or microcode specific to the underlying hardware andcontrols operating system access to the system hardware.

A guest instruction 40 that is obtained, translated and executed may be,for instance, one of the instructions described herein. The instruction,which is of one architecture (e.g., the z/Architecture), is fetched frommemory, translated and represented as a sequence of native instructions46 of another architecture (e.g., PowerPC, pSeries, Intel, etc.). Thesenative instructions are then executed.

One or more aspects may relate to cloud computing.

It is understood in advance that although this disclosure includes adetailed description on cloud computing, implementation of the teachingsrecited herein are not limited to a cloud computing environment. Rather,embodiments of the present invention are capable of being implemented inconjunction with any other type of computing environment now known orlater developed.

Cloud computing is a model of service delivery for enabling convenient,on-demand network access to a shared pool of configurable computingresources (e.g. networks, network bandwidth, servers, processing,memory, storage, applications, virtual machines, and services) that canbe rapidly provisioned and released with minimal management effort orinteraction with a provider of the service. This cloud model may includeat least five characteristics, at least three service models, and atleast four deployment models.

Characteristics are as follows:

On-demand self-service: a cloud consumer can unilaterally provisioncomputing capabilities, such as server time and network storage, asneeded automatically without requiring human interaction with theservice's provider.

Broad network access: capabilities are available over a network andaccessed through standard mechanisms that promote use by heterogeneousthin or thick client platforms (e.g., mobile phones, laptops, and PDAs).

Resource pooling: the provider's computing resources are pooled to servemultiple consumers using a multi-tenant model, with different physicaland virtual resources dynamically assigned and reassigned according todemand. There is a sense of location independence in that the consumergenerally has no control or knowledge over the exact location of theprovided resources but may be able to specify location at a higher levelof abstraction (e.g., country, state, or datacenter).

Rapid elasticity: capabilities can be rapidly and elasticallyprovisioned, in some cases automatically, to quickly scale out andrapidly released to quickly scale in. To the consumer, the capabilitiesavailable for provisioning often appear to be unlimited and can bepurchased in any quantity at any time.

Measured service: cloud systems automatically control and optimizeresource use by leveraging a metering capability at some level ofabstraction appropriate to the type of service (e.g., storage,processing, bandwidth, and active user accounts). Resource usage can bemonitored, controlled, and reported providing transparency for both theprovider and consumer of the utilized service.

Service Models are as follows:

Software as a Service (SaaS): the capability provided to the consumer isto use the provider's applications running on a cloud infrastructure.The applications are accessible from various client devices through athin client interface such as a web browser (e.g., web-based email). Theconsumer does not manage or control the underlying cloud infrastructureincluding network, servers, operating systems, storage, or evenindividual application capabilities, with the possible exception oflimited user-specific application configuration settings.

Platform as a Service (PaaS): the capability provided to the consumer isto deploy onto the cloud infrastructure consumer-created or acquiredapplications created using programming languages and tools supported bythe provider. The consumer does not manage or control the underlyingcloud infrastructure including networks, servers, operating systems, orstorage, but has control over the deployed applications and possiblyapplication hosting environment configurations.

Infrastructure as a Service (IaaS): the capability provided to theconsumer is to provision processing, storage, networks, and otherfundamental computing resources where the consumer is able to deploy andrun arbitrary software, which can include operating systems andapplications. The consumer does not manage or control the underlyingcloud infrastructure but has control over operating systems, storage,deployed applications, and possibly limited control of select networkingcomponents (e.g., host firewalls).

Deployment Models are as follows:

Private cloud: the cloud infrastructure is operated solely for anorganization. It may be managed by the organization or a third party andmay exist on-premises or off-premises.

Community cloud: the cloud infrastructure is shared by severalorganizations and supports a specific community that has shared concerns(e.g., mission, security requirements, policy, and complianceconsiderations). It may be managed by the organizations or a third partyand may exist on-premises or off-premises.

Public cloud: the cloud infrastructure is made available to the generalpublic or a large industry group and is owned by an organization sellingcloud services.

Hybrid cloud: the cloud infrastructure is a composition of two or moreclouds (private, community, or public) that remain unique entities butare bound together by standardized or proprietary technology thatenables data and application portability (e.g., cloud bursting forloadbalancing between clouds).

A cloud computing environment is service oriented with a focus onstatelessness, low coupling, modularity, and semantic interoperability.At the heart of cloud computing is an infrastructure comprising anetwork of interconnected nodes.

Referring now to FIG. 28, illustrative cloud computing environment 50 isdepicted. As shown, cloud computing environment 50 comprises one or morecloud computing nodes 10 with which local computing devices used bycloud consumers, such as, for example, personal digital assistant (PDA)or cellular telephone 54A, desktop computer 54B, laptop computer 54C,and/or automobile computer system 54N may communicate. Nodes 10 maycommunicate with one another. They may be grouped (not shown) physicallyor virtually, in one or more networks, such as Private, Community,Public, or Hybrid clouds as described hereinabove, or a combinationthereof. This allows cloud computing environment 50 to offerinfrastructure, platforms and/or software as services for which a cloudconsumer does not need to maintain resources on a local computingdevice. It is understood that the types of computing devices 54A-N shownin FIG. 28 are intended to be illustrative only and that computing nodes10 and cloud computing environment 50 can communicate with any type ofcomputerized device over any type of network and/or network addressableconnection (e.g., using a web browser).

Referring now to FIG. 29, a set of functional abstraction layersprovided by cloud computing environment 50 (FIG. 28) is shown. It shouldbe understood in advance that the components, layers, and functionsshown in FIG. 29 are intended to be illustrative only and embodiments ofthe invention are not limited thereto. As depicted, the following layersand corresponding functions are provided:

Hardware and software layer 60 includes hardware and softwarecomponents. Examples of hardware components include mainframes 61; RISC(Reduced Instruction Set Computer) architecture based servers 62;servers 63; blade servers 64; storage devices 65; and networks andnetworking components 66. In some embodiments, software componentsinclude network application server software 67 and database software 68.

Virtualization layer 70 provides an abstraction layer from which thefollowing examples of virtual entities may be provided: virtual servers71; virtual storage 72; virtual networks 73, including virtual privatenetworks; virtual applications and operating systems 74; and virtualclients 75.

In one example, management layer 80 may provide the functions describedbelow. Resource provisioning 81 provides dynamic procurement ofcomputing resources and other resources that are utilized to performtasks within the cloud computing environment. Metering and Pricing 82provide cost tracking as resources are utilized within the cloudcomputing environment, and billing or invoicing for consumption of theseresources. In one example, these resources may comprise applicationsoftware licenses. Security provides identity verification for cloudconsumers and tasks, as well as protection for data and other resources.User portal 83 provides access to the cloud computing environment forconsumers and system administrators. Service level management 84provides cloud computing resource allocation and management such thatrequired service levels are met. Service Level Agreement (SLA) planningand fulfillment 85 provide pre-arrangement for, and procurement of,cloud computing resources for which a future requirement is anticipatedin accordance with an SLA.

Workloads layer 90 provides examples of functionality for which thecloud computing environment may be utilized. Examples of workloads andfunctions which may be provided from this layer include: mapping andnavigation 91; software development and lifecycle management 92; virtualclassroom education delivery 93; data analytics processing 94;transaction processing 95; and register processing 96.

The present invention may be a system, a method, and/or a computerprogram product at any possible technical detail level of integration.The computer program product may include a computer readable storagemedium (or media) having computer readable program instructions thereonfor causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, configuration data for integrated circuitry, oreither source code or object code written in any combination of one ormore programming languages, including an object oriented programminglanguage such as Smalltalk, C++, or the like, and procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The computer readable program instructions may executeentirely on the user's computer, partly on the user's computer, as astand-alone software package, partly on the user's computer and partlyon a remote computer or entirely on the remote computer or server. Inthe latter scenario, the remote computer may be connected to the user'scomputer through any type of network, including a local area network(LAN) or a wide area network (WAN), or the connection may be made to anexternal computer (for example, through the Internet using an InternetService Provider). In some embodiments, electronic circuitry including,for example, programmable logic circuitry, field-programmable gatearrays (FPGA), or programmable logic arrays (PLA) may execute thecomputer readable program instructions by utilizing state information ofthe computer readable program instructions to personalize the electroniccircuitry, in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

In addition to the above, one or more aspects may be provided, offered,deployed, managed, serviced, etc. by a service provider who offersmanagement of customer environments. For instance, the service providercan create, maintain, support, etc. computer code and/or a computerinfrastructure that performs one or more aspects for one or morecustomers. In return, the service provider may receive payment from thecustomer under a subscription and/or fee agreement, as examples.Additionally or alternatively, the service provider may receive paymentfrom the sale of advertising content to one or more third parties.

In one aspect, an application may be deployed for performing one or moreembodiments. As one example, the deploying of an application comprisesproviding computer infrastructure operable to perform one or moreembodiments.

As a further aspect, a computing infrastructure may be deployedcomprising integrating computer readable code into a computing system,in which the code in combination with the computing system is capable ofperforming one or more embodiments.

As yet a further aspect, a process for integrating computinginfrastructure comprising integrating computer readable code into acomputer system may be provided. The computer system comprises acomputer readable medium, in which the computer medium comprises one ormore embodiments. The code in combination with the computer system iscapable of performing one or more embodiments.

Although various embodiments are described above, these are onlyexamples. For example, computing environments of other architectures canbe used to incorporate and use one or more embodiments. Further,different instructions or operations may be used. Additionally,different registers may be used and/or other types of indications (otherthan register numbers) may be specified. Many variations are possible.

Further, other types of computing environments can benefit and be used.As an example, a data processing system suitable for storing and/orexecuting program code is usable that includes at least two processorscoupled directly or indirectly to memory elements through a system bus.The memory elements include, for instance, local memory employed duringactual execution of the program code, bulk storage, and cache memorywhich provide temporary storage of at least some program code in orderto reduce the number of times code must be retrieved from bulk storageduring execution.

Input/Output or I/O devices (including, but not limited to, keyboards,displays, pointing devices, DASD, tape, CDs, DVDs, thumb drives andother memory media, etc.) can be coupled to the system either directlyor through intervening I/O controllers. Network adapters may also becoupled to the system to enable the data processing system to becomecoupled to other data processing systems or remote printers or storagedevices through intervening private or public networks. Modems, cablemodems, and Ethernet cards are just a few of the available types ofnetwork adapters.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a”, “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willbe further understood that the terms “comprises” and/or “comprising”,when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below, if any, areintended to include any structure, material, or act for performing thefunction in combination with other claimed elements as specificallyclaimed. The description of one or more embodiments has been presentedfor purposes of illustration and description, but is not intended to beexhaustive or limited to in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the art. Theembodiment was chosen and described in order to best explain variousaspects and the practical application, and to enable others of ordinaryskill in the art to understand various embodiments with variousmodifications as are suited to the particular use contemplated.

1. A computer program product for facilitating processing within acomputing environment, the computer program product comprising: acomputer readable storage medium readable by a processing circuit andstoring instructions for performing a method comprising: predicting,using a processor of the computing environment, a predicted value to beused in register-indirect branching; storing the predicted value in aselected location accessible to one or more instructions of thecomputing environment, the storing being performed concurrently toprocessing of a register-indirect branch, and the selected locationbeing in addition to another location used to store an instructionaddress; and using the predicted value in speculative processing thatincludes the register-indirect branch.
 2. The computer program productof claim 1, wherein the predicted value comprises a target address to beused by an instruction fetch.
 3. The computer program product of claim2, wherein the using comprises redirecting the instruction fetch to thetarget address.
 4. The computer program product of claim 1, wherein theselected location comprises a hardware register.
 5. The computer programproduct of claim 1, wherein the method further comprises: determiningwhether the predicted value is accurate; and performing recovery, basedon determining the predicted value is inaccurate.
 6. The computerprogram product of claim 5, wherein the recovery comprises: flushing oneor more instructions executed subsequent to the using; and replacing thepredicted value with a non-predicted value.
 7. The computer programproduct of claim 6, wherein the replacing comprises using one or morerename registers to provide the non-predicted value.
 8. The computerprogram product of claim 1, wherein the storing comprises: allocating aregister to hold the predicted value; and copying the predicted value tothe register allocated to hold the predicted value.
 9. The computerprogram product of claim 8, wherein the register is a rename registerallocated for a logical register assigned to hold the predicted value.10. The computer program product of claim 9, wherein the method furthercomprises marking the rename register as available.
 11. A computersystem for facilitating processing within a computing environment, thecomputer system comprising: a memory; and a processor in communicationwith the memory, wherein the computer system is configured to perform amethod, said method comprising: predicting a predicted value to be usedin register-indirect branching; storing the predicted value in aselected location accessible to one or more instructions of thecomputing environment, the storing being performed concurrently toprocessing of a register-indirect branch, and the selected locationbeing in addition to another location used to store an instructionaddress; and using the predicted value in speculative processing thatincludes the register-indirect branch.
 12. The computer system of claim11, wherein the predicted value comprises a target address to be used byan instruction fetch.
 13. The computer system of claim 11, wherein theselected location comprises a hardware register.
 14. The computer systemof claim 11, wherein the method further comprises: determining whetherthe predicted value is accurate; and performing recovery, based ondetermining the predicted value is inaccurate.
 15. The computer systemof claim 11, wherein the storing comprises: allocating a register tohold the predicted value; and copying the predicted value to theregister allocated to hold the predicted value. 16-20. (canceled)